Surface Recovery of the CXRS First Mirror of EAST

A radio frequency (RF) capacitively coupled plasma was used to clean the nonplane large size first mirror (FM) used in the charge exchange recombination system (CXRS) of the Experimental Advanced Superconducting Tokamak (EAST). The mirror is made of 316L stainless steel (SS) and has a curved surface with a radius of 1600 mm and dimensions of 300 mm × 80 mm × 40 mm. Before cleaning, the FM was exposed to the plasma in 12 499 plasma discharges with a total plasma duration time of 86 036 s during three experimental campaigns from 2014 to 2016. An inhomogeneous deposition layer, consisting of carbon (C), oxygen (O), silicon (Si), tungsten (W), and molybdenum (Mo) with deposit particles sizes up to tens of micrometers was detected afterward. To recover the FM surface, argon (Ar) plasma cleaning was performed for 187.3 h in the laboratory. The inhomogeneous deposition was afterward not visible and uniformly removed. Scanning electron microscopy (SEM) and electron energy dispersion spectroscopy (EDS) results indicated that the micromorphology was developed during the cleaning, and few residual particles consisting of C and O also remained and covered about 5.3% of the cleaned mirror surface. The reflectivity of the FM was recovered from 15% to 68% at the wavelength of 532 nm, which was close to the initial preexposure value of 71.4%. A negligible difference of the recovered reflectivity along the FM surface was obtained which demonstrated the cleaning effectiveness and homogeneity of RF plasma cleaning for nonplane large mirrors. To prolong the FM lifetime, RF plasma could, therefore, be considered as one of the possible way for in situ cleaning.